The last two Purge and Trap (P&T) settings we will discuss in this blog series might be the most important for moisture control in this volatile organic compound (VOC) analysis: desorb and bake settings.
We can adjust our desorb time to help reduce the amount of moisture we put onto the gas chromatograph (GC) and mass spectrometer (MS). A longer desorb time will remove more of compounds from the analytical trap and send them to the GC, but this includes water, leading to a longer water peak in the total ion chromatogram. If the desorb time is shortened, less of that analyte material is transferred to the GC, but we will have a much shorter water peak, giving the compound peaks better shape and resolution. This gives us an opportunity for method optimization with the desorb settings both in time and temperature.
A rapid increase in temperature during desorb pre-heat can drive moisture out of the sample; the final temperature of desorb ensures that moisture stays out of the gas flow and that all the target analytes are transferred to the GC. Our research shows that compounds of interest are out within one to two minutes of desorb. That amount of time will give us enough compound transfer to the GC to give us the sensitivity that we need for this analysis. Beyond one or two minutes of desorb, you will only introduce more water vapor into the system. So, if the method allows, shortening the desorb time can significantly reduce the amount of moisture introduced to the GC.
Baking removes all of the moisture that has been accumulated in the P&T system during sample analysis. This ensures that it can continue to remove water from each sample to follow. As an added bonus, baking also helps prevent carryover from sample to sample. The mass flow controller (MFC) on Tekmar’s P&T concentrators allows for improved bake mode flow control. Faster flow rates and higher temperatures ultimately lead to less bake time and increased sample throughput.
Areas of method optimization for bake settings would be time and temperature. The temperature used for bake is dependent on the analytical trap. We want the temperature to be high enough to be effective, but we do not want to overheat and degrade the packing material, causing a breakdown of the trapping adsorbents. Depending on the method requirements and analytical trap, you can trust that the default bake settings in the TekLink™ software will provide an adequate and efficient bake for your P&T system.
This is a four part series discussing moisture control. For the final part of this series, we will explore how to optimize the method parameters on the GC-MS side of the analysis for moisture control.
- Part 1: Why is water vapor bad for VOC Analysis?
- Part 2: Ways to optimize moisture control in the Purge and Trap
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